1. Field of the Invention
The invention relates to a new class of aminoamide acrylate polymers useful as hot melt adhesives which are curable by actinic radiation.
2. Brief Description of the Prior Art
Polyamides are a well known class of thermoplastic polymer. Most polyamides offer no practicable means by which they can be cured to thermoset polymers. A known subclass of polyamides are the aminoamide polymers which have free (i.e. non-acylated) amino groups. Certain of these are useful hot melt adhesives. However, if they are used in a non-crosslinked thermoplastic mode, they are subject to cold flow, remelting, solvent attack, degradation by moisture, and other modes of deterioration. Means are known for crosslinking aminoamides, in fact they are often used themselves as crosslinking agents for epoxy resins, but such crosslinking generally requires a "two package" system, the aminoamide resin being one package, and a co-reactive resin, typically an epoxy resin, as the other package. Such "two package" systems are extensively used, but they have the shortcomings of requiring two containers, measuring and mixing at the point of use, limited working time after the mixing, and they generally require substantial time and/or heating to effect the partial cure to give green strength. Even further time and/or heating is required for complete cure.
It is desirable to have a hot melt adhesive with good gripping power as first applied, but with curing capabilities for greatly improved adhesion, cohesion, resistance to heat, solvents, moisture, and other stresses. It is further desirable to have such an adhesive as a one-package product. It is further desirable to have a long storage and working time but a short cure time.
One known approach to curing hot melt adhesives is to incorporate isocyanate groups so that they are moisture-curable. However such products are prone to premature curing and moreover, since the curing reaction evolves carbon dioxide, they are prone to be weakened by the voids caused by the evolved gas.
It has been known further to blend acrylate polymers into certain polyamides for improved properties such as hot melt adhesive characteristics, but these acrylate polymers have the acrylate group already polymerized and thus rendered nonfunctional, so that such blends were neither chemically linked by covalent bonds nor subject to post-cure to convert them to the thermoset condition.
A number of classes of radiation curable acrylates are known to the art, such as the acrylates of polyurethanes described in U.S. Pat. No. 4,153,776; Friedlander et al. (1979) and the acrylate derivatives of polycaprolactones as in U.S. Pat. No. 3,700,643 (1972); Smith et al. but these do not have in general the hot melt adhesive character of the aminoamide resins. Moreover, they tend to be more costly than aminoamide resins which can be made from byproducts (tall oil, for example).
Our invention provides an improvement over the prior art in that it makes possible a one package system, with good storage properties, combining good hot melt adhesive properties with the ability to cure very rapidly with radiation to a thermoset (crosslinked) product. Thus, our invention can provide hot melt adhesives, varnishes, lacquers, inks and encapsulants which have good initial (green) strength properties upon cooling, but, in addition, are rapidly curable to a crosslinked state where they have improved adhesive strength and are resistant to moisture damage, cold flow, remelting (or softening with heat), blocking, and solvent attack. These characteristics are especially valuable in hot melt adhesives which must perform under challenging conditions of temperature, humidity, and mechanical stress, such as in bonding in outdoor furniture, automotive products and building materials.